Determination of Heat Transfer Coefficient in Cooling‐freezing Tunnels Using Experimental Time–temperature Data

Abstract

In industrial freezing tunnels, knowledge of the spatial distribution of convective heat transfer (h) is crucial in predicting process times. The objective of this study was to develop an appropriate and simple methodology to determine the spatial distribution of h values inside tunnels. A dedicated device, consisting of a rectangular metal tank filled with an alcoholic solution, with a temperature sensor inserted in its interior, was used as a model system. The transient temperatures of the system (tank solution) were used to calculate h values at different positions inside the tunnels. The main result of this paper is the experimental method itself, which is a reproducible and reliable procedure for determining heat transfer coefficients inside industrial tunnels. The method is sensitive enough to determine spatial heterogeneities and is versatile with respect to the system geometry, which makes it a useful and simple tool to carry out diagnostic tests in industrial plants. PRACTICAL APPLICATIONS In cooling/freezing tunnels, it is important to have convective heat transfer conditions that are spatially homogeneous, leading to homogeneous cooling/freezing processes. This is not obvious in industrial tunnels because of the arrangement of packages and fan positions. Tunnel operational conditions and layout lead to a specific airflow pattern, creating zones of high and low air flux that leads to zones of high and low convective heat transfer (h) values. This is the case for industrial tunnels used to freeze pack poultry cuts. The method proposed in this work is a useful tool to detect h value heterogeneities and to determine the average h value of preexisting tunnels. Such diagnoses can support decisions regarding layout changes or changes in the amount and positions of the fans in a tunnel. Computational fluid dynamics can also be used, but this provides more realistic data if actual h values are used.

Experimental results and modeling of poultry carcass cooling by water immersion

“…When the evaporative cooling was taken in account (from experimental mass loss data during cooling) to correct the values of the heat transfer coefficients, they found coefficient values of about 19 Wm −2 K −1 for 0.42 mass transfer factor. Another versatile experimental device was reported by Santos et al (2007) to determine heat transfer coefficients inside air blast tunnels. This device could be adapted for use in immersion tanks.…”

Numerical Simulation of Experimental Freezing Process of Ground Meat Cylinders

“…Then, local coefficients obtained at each angular point, dependent time and location, are included in the freezing modeling for each sample. The calculation of the local heat transfer coefficients was performed by the discrete thermal capacitance method for the copper cylinders, verifying that the conduction thermal resistance was negligible compared to the external forced convective heat transfer resistance by the Biot number (Bi<<1) (Santos et al, 2007). This simplification is justified when the external thermal resistance is much higher than the internal thermal resistance (cylinder 1, Bi=0.0041; cylinder 2, Bi=0.0026; cylinder 3, Bi=0.0041).…”

“…The heat transfer coefficient is very important in the evaluation and subsequent use in predicting temperature distributions and freezing time, as in the design of blast freezers and refrigerators (Smalea et al, 2006;Santos et al, 2007). The heat transfer coefficient depends on the fluid properties, food geometry, flow rate, and process time (Amarante & Lanoisellé, 2005).…”

“…In addition, a main aspect is food geometric shape (slab, cylinder and sphere), which can be solved with analytical and numerical methods (Cleland et al, 1987, Hossain et al, 1992. For that reason, the knowledge of the heat transfer coefficient and freezing time is needed in control and optimization of industrial plant freezing processes (Smalea et al, 2006;Santos et al, 2007).…”

Influence of secondary packing on the freezing time of chiken meat in air blast freezing tunnels

“…Poultry and other meat products are generally frozen in forced air blast tunnels where the circulating air flows at 3 to 8 m/s, at temperatures ranging from -35 to -45 °C. Those freezing tunnels present diverse geometries and air flow heterogeneities (SANTOS et al, 2007), but the heat transfer from the product to the air takes place always in transient state (PARDI et al, 2001;RESENDE et al, 2002a).…”